Adenine nucleotide metabolism by chondrocytes in vitro: role of ATP in chondrocyte maturation and matrix mineralization

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

Inhibition of lung cancer cells and Ras/Raf/MEK/ERK signal transduction by ectonucleoside triphosphate phosphohydrolase-7 (ENTPD7)

Background

The aim of this study was to investigate the effects and mechanisms of ectonucleoside triphosphate phosphohydrolase-7 (ENTPD7) on lung cancer cells.

Methods

The expression characteristics of ENTPD7 and its effect on the survival of lung cancer patients were analyzed by referring to The Cancer Genome Atlas (TCGA). Streptavidin-peroxidase (SP) staining was performed to detect the ENTPD7 protein in tumor tissues and adjacent tissues. Plasmid transfection technology was also applied to silence ENTPD7 gene. Crystal violet staining and flow cytometry were performed to determine cell proliferation and apoptosis. Tumor-bearing nude mice model was established to investigate the effect of sh-ENTPD7 on tumors.

Results

The results showed that patients with low levels of ENTPD7 had higher survival rates. ENTPD7 was up-regulated in lung cancer tissues and cells. Down-regulation of the expression of ENTPD7 inhibited proliferation but promoted apoptosis of lung cancer cell. Silencing ENTPD7 also inhibited the expression levels of Ras and Raf proteins and the phosphorylation of mitogen-activated protein kinase (MEK) and extracellular signal-regulated kinase (ERK). Tumor-bearing nude mice experiments showed that silencing ENTPD7 had an inhibitory effect on lung cancer cells.

Conclusions

ENTPD7 was overexpressed in lung cancer cells. Down-regulating ENTPD7 could inhibit lung cancer cell proliferation and promote apoptosis via inhibiting the Ras/Raf/MEK/ERK pathway.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

The P2Y13 receptor regulates phosphate metabolism and FGF-23 secretion with effects on skeletal development

Purinergic signaling mediates many cellular processes, including embryonic development and regulation of endocrine signaling. The ADP P2Y13 receptor is known to regulate bone and stem cells activities, although relatively little is known about its role in bone development. In this study we demonstrate, using contemporary techniques, that deletion of the P2Y13 receptor results in an age-dependent skeletal phenotype that is governed by changes in phosphate metabolism and hormone levels. Neonatal and postnatal (2 wk) P2Y13 receptor-knockout (KO) mice were indistinguishable from their wild-type (WT) littermate controls. A clear bone phenotype was observed in young (4-wk-old) KO mice compared WT controls, with 14% more trabecular bone, 35% more osteoblasts, 73% fewer osteoclasts, and a 17% thicker growth plate. Mature (>10 wk of age) KO mice showed the opposite bone phenotype, with 14% less trabecular bone, 22% fewer osteoblasts, and 10% thinner growth plate. This age-dependent phenotype correlated with serum fibroblast growth factor-23 (FGF-23) and phosphorus levels that were 65 and 16% higher, respectively, in young KO mice but remained unchanged in mature mice. These findings provide novel insights for the role of the P2Y13 receptor in skeletal development via coordination with hormonal regulators of phosphate homeostasis.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Active creatine kinase is present in matrix vesicles isolated from femurs of chicken embryo: Implications for bone mineralization

Proteomic analysis of matrix vesicles (MVs) isolated from 17-day-old chicken embryo femurs revealed the presence of creatine kinase. In this report we identified the enzyme functionally and suggest that the enzyme may participate in the synthesis of ATP from ADP and phosphocreatine within the lumen of these organelles. Then, ATP is converted by nucleotide hydrolyzing enzymes such as Na(+), K(+)-ATPase, protein kinase C, or alkaline phosphatase to yield inorganic phosphate (P(i)), a substrate for mineralization. Alternatively, ATP can be hydrolyzed by a nucleoside triphosphate pyrophosphatase phosphodiesterase 1 producing inorganic pyrophosphate (PP(i)), a mineralization inhibitor. In addition, immunochemical evidence indicated that VDAC 2 is present in MVs that may serve as a transporter of nucleotides from the extracellular matrix. We discussed the implications of ATP production and hydrolysis by MVs as regulatory mechanisms for mineralization.

Transduction of anti-cell death protein FNK suppresses graft degeneration after autologous cylindrical osteochondral transplantation

This study shows that artificial super antiapoptotic FNK protein fused with a protein transduction domain (PTD-FNK) maintains the quality of osteochondral transplant by preventing chondrocyte death. Cylindrical osteochondral grafts were obtained from enhanced green fluorescent protein (EGFP)-expressing transgenic rats, in which living chondrocytes express green fluorescence, and submerged into medium containing PTD-FNK, followed by transplantation into cartilage defects of wild-type rats by impact insertion simulating autologous transplantation. The tissues were histologically evaluated by hematoxylin-eosin and Safranin-O staining. At 1 week, chondrocyte alignment was normal in the PTD-FNK treatment group, whereas all grafts without PTD-FNK treatment showed mixed cluster cell distribution. At 4 weeks, all grafts with PTD-FNK treatment showed almost normal matrix, whereas two grafts without PTD-FNK treatment showed fibrocartilage. Notably, all grafts with PTD-FNK retained high intensity of Safranin-O staining, but all grafts without PTD-FNK largely lost Safranin-O staining. PTD-FNK significantly suppressed a decrease in the survival rate and the density of EGFP-positive cells at 1 and 2 weeks, and this tendency continued at 4 weeks. The results of terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-nick end-labeling staining showed that PTD-FNK inhibited cell death, indicating that PTD-FNK protects chondrocyte death and suppresses graft degeneration.

Autophagy: a new phase in the maturation of growth plate chondrocytes is regulated by HIF, mTOR and AMP kinase

The overall goal of the investigation was to examine autophagy in the growth plate and to ascertain how this process was regulated. Herein, we show that in the postmitotic maturing zone of the growth plate, chondrocytes express an autophagic phenotype. This robust and particulate immunohistochemical response provides direct evidence that autophagy is a new and transient stage in the chondrocyte maturation pathway. We found that induction of autophagy was regulated by mTOR, a sensor of cellular metabolism. When mTOR was inhibited, changes in LC3 fluorescence indicated that this kinase regulated development of the autophagic state. To determine if AMP kinase was required for chondrocyte autophagy, we suppressed its expression in N1511 cells using siRNA technology. When these cells were serum starved, a condition that triggers autophagy, there was no change in LC3 distribution. This result confirmed that AMP kinase was required for the induction of the autophagic response. Based on the 2 studies described above, and our previous observation that HIF-1 is required for the induction of autophagy, we put forward the hypothesis that autophagy is regulated by the activities of AMP kinase and mTOR in a HIF-1-dependent manner. Once autophagy is activated, the postmitotic chondrocytes would be expected to remain viable in their unique microenvironment and complete their life cycle.

Proteome analysis of matrix vesicles isolated from femurs of chicken embryo

Matrix vesicles (MVs) are extracellular organelles that initiate mineral formation, accumulating inorganic phosphate (P(i)) and calcium leading to the formation of hydroxyapatite (HA) crystals, the main mineral component of bones. MVs are produced during bone formation, as well as during the endochondral calcification of cartilage. MVs are released into the extracellular matrix from osseous cells such as osteoblasts and hypertrophic chondrocytes. In this report, using 1-D SDS-PAGE, in-gel tryptic digestion and an LC-MS-MS/MS protein identification protocol, we characterized the proteome of MVs isolated from chicken embryo (Gallus gallus) bones and cartilage. We identified 126 gene products, including proteins related to the extracellular matrix and ion transport, as well as enzymes, cytoskeletal, and regulatory proteins. Among the proteins recognized for the first time in MVs were aquaporin 1, annexin A1 (AnxA1), AnxA11, glycoprotein HT7, G(i) protein alpha2, and scavenger receptor type B. The pathways for targeting the identified proteins into MVs and their particular functions in the biomineralization process are discussed. Obtaining a knowledge of the functions and roles of these proteins during embryonic mineralization is a prerequisite for the overall understanding of the initial mineral formation mechanisms.

P2Y nucleotide receptor signaling through MAPK/ERK is regulated by extracellular matrix: involvement of beta3 integrins

Extracellular matrix influences cell behavior through receptors such as integrins and through transmission of mechanical forces. Nucleotides are released in response to mechanical stimuli and bind to P2 nucleotide receptors. As chondrocytes are subjected to frequent mechanical stimulation within a rich extracellular matrix, they are an excellent model for studying integration of signals induced by matrix and nucleotides. We investigated signaling of G protein-coupled P2Y receptors to MAPK/ERK and how this is influenced by matrix. Rat articular chondrocytes expressed transcripts for P2Y1, P2Y2, P2Y4, and P2Y6 receptors and responded to extracellular nucleotides by transient elevation of cytosolic calcium and MAPK/ERK phosphorylation. ERK1/2 activation was suppressed by the protein kinase C (PKC) inhibitors bisindolylmaleimide I and rottlerin, and by the phospholipase D inhibitor 1-butanol. Thus, nucleotides stimulate P2Y receptors to activate ERK1/2 through a mechanism dependent on PKC and phospholipase D. We next examined the involvement of integrins. Both an RGD-containing pentapeptide and a beta3 integrin blocking antibody, but not a beta1 integrin blocking antibody, abolished nucleotide-induced ERK1/2 phosphorylation. Moreover, chondrocytes adhering to fibronectin (which binds to beta1 and beta3 containing integrins in an RGD-dependent manner) displayed prolonged ERK1/2 signaling compared to cells grown on type I or II collagen (which bind to beta1-containing integrins in an RGD-independent manner). In conclusion, P2Y receptor signaling through ERK1/2 is gated selectively by matrix proteins. Thus, nucleotides released in response to mechanical stimulation will have differing effects on cell function due to changes in the composition of the extracellular matrix during development and disease.

Spontaneous oscillation and mechanically induced calcium waves in chondrocytes

The characteristics of spontaneous calcium (Ca(2+)) oscillation and mechanically induced Ca(2+) waves in articular chondrocytes were studied. In some, but not all, chondrocytes in sliced cartilage and primary cultures, we observed spontaneous oscillation of intracellular Ca(2+) that never spread to adjacent cells. In contrast, a mechanical stimulus to a single cell by touching with a glass rod induced an increase of intracellular Ca(2+) that spread to neighboring cells in a wave-like manner, even though there was no physical contact between the cells. This indicated the release of some paracrine factor from the mechanically stimulated cells. Application of ultrasonic vibration also induced an oscillation of intracellular Ca(2+). The application of a uridine 5'-triphosphate (UTP), UTP, induced a transient increase in intracellular Ca(2+) and the release of adenosine 5'-triphosphate (ATP) in cultured chondrocytes. A P2 receptor antagonist (suramin) and blockers of Cl(-) channels, niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), reduced the UTP-induced ATP release. The results indicated that Cl(-) channels were involved in the extracellular release of ATP following mechanical or P2Y receptor stimulation. Thus, ATP stimulation of P2Y receptors elicits an increase in intracellular Ca(2+), triggering further release of ATP from adjacent cells, thereby expanding the Ca(2+) wave in chondrocytes.

Phosphodiesterase activity of alkaline phosphatase in ATP-initiated Ca(2+) and phosphate deposition in isolated chicken matrix vesicles

Inorganic pyrophosphate is a potent inhibitor of bone mineralization by preventing the seeding of calcium-phosphate complexes. Plasma cell membrane glycoprotein-1 and tissue nonspecific alkaline phosphatase were reported to be antagonistic regulators of mineralization toward inorganic pyrophosphate formation (by plasma cell membrane glycoprotein-1) and degradation (by tissue nonspecific alkaline phosphatase) under physiological conditions. In addition, they possess broad overlapping enzymatic functions. Therefore, we examined the roles of tissue nonspecific alkaline phosphatase within matrix vesicles isolated from femurs of 17-day-old chick embryos, under conditions where these both antagonistic and overlapping functions could be evidenced. Addition of 25 microM ATP significantly increased duration of mineralization process mediated by matrix vesicles, while supplementation of mineralization medium with levamisole, an alkaline phosphatase inhibitor, reduces the ATP-induced retardation of mineral formation. Phosphodiesterase activity of tissue nonspecific alkaline phosphatase for bis-p-nitrophenyl phosphate was confirmed, the rate of this phosphodiesterase activity is in the same range as that of phosphomonoesterase activity for p-nitrophenyl phosphate under physiological pH. In addition, tissue nonspecific alkaline phosphatase at pH 7.4 can hydrolyze ADPR. On the basis of these observations, it can be concluded that tissue nonspecific alkaline phosphatase, acting as a phosphomonoesterase, could hydrolyze free phosphate esters such as pyrophosphate and ATP, while as phosphodiesterase could contribute, together with plasma cell membrane glycoprotein-1, in the production of pyrophosphate from ATP.

Emerging neuroskeletal signalling pathways: a review

Recent work has demonstrated that neurotransmitters, signalling molecules primarily associated with the nervous system, can have profound effects on the skeleton. Bone cells express a broad range of neurotransmitter receptors and transporters, and respond to receptor activation by initiating diverse intracellular signalling pathways, which modulate cellular function. Evidence of neuronal innervation in skeletal tissues, neurotransmitter release directly from bone cells and functional effects of pharmacological manipulation support the existence of a complex and functionally significant neurotransmitter-mediated signalling network in bone. This review aims to concisely summarise our current understanding of how neurotransmitters affect the skeletal system, focusing on their origin, cellular targets and functional effects in bone.

Regulation of bone resorption and formation by purines and pyrimidines

Growing evidence suggests that extracellular nucleotides, signalling through P2 receptors, might play important roles in the regulation of bone and cartilage metabolism. ATP and other nucleotides can exert impressive stimulatory effects on the formation and activity of osteoclasts (bone-resorbing cells) in addition to inhibiting bone formation by osteoblasts. In this review, the current understanding of the actions of nucleotides on skeletal cells and the probable receptor subtypes involved are discussed.

Chick embryo anchored alkaline phosphatase and mineralization process in vitro

Bone alkaline phosphatase with glycolipid anchor (GPI-bALP) from chick embryo femurs in a medium without exogenous inorganic phosphate, but containing calcium and GPI-bALP substrates, served as in vitro model of mineral formation. The mineralization process was initiated by the formation of inorganic phosphate, arising from the hydrolysis of a substrate by GPI-bALP. Several mineralization media containing different substrates were analysed after an incubation time ranging from 1.5 h to 144 h. The measurements of Ca/Pi ratio and infrared spectra permitted us to follow the presence of amorphous and noncrystalline structures, while the analysis of X-ray diffraction data allowed us to obtain the stoichiometry of crystals. The hydrolysis of phosphocreatine, glucose 1-phosphate, glucose 6-phosphate, glucose 1,6-bisphosphate by GPI-bALP produced hydroxyapatite in a manner similar to that of beta-glycerophosphate. Several distinct steps in the mineral formation were observed. Amorphous calcium phosphate was present at the onset of the mineral formation, then poorly formed hydroxyapatite crystalline structures were observed, followed by the presence of hydroxyapatite crystals after 6-12 h incubation time. However, the hydrolysis of either ATP or ADP, catalysed by GPI-bALP in calcium-containing medium, did not lead to the formation of any hydroxyapatite crystals, even after 144 h incubation time, when hydrolysis of both nucleotides was completed. In contrast, the hydrolysis of AMP by GPI-bALP led to the appearance of hydroxyapatite crystals after 12 h incubation time. The hydroxyapatite formation depends not only on the ability of GPI-bALP to hydrolyze the organic phosphate but also on the nature of substrates affecting the nucleation process or producing inhibitors of the mineralization.

Extracellular nucleotides, cartilage stress, and calcium crystal formation

Nucleotides are released by chondrocytes at rest and in response to mechanical stimulation. Extracellular nucleotides are metabolized by a variety of ectoenzymes, producing free phosphate (Pi) or pyrophosphate (PPi) and promoting matrix mineralization. Ectoenzymes are differentially localized in cartilage and may be co-released with nucleotides during mechanical stimulation. Extracellular nucleotides can also serve as substrates and/or modulators of enzymes such as tissue transglutaminase and ecto-protein kinases that modify matrix proteins and regulate crystal deposition or growth. Understanding the evolution of osteoarthritis and calcium crystal deposition diseases will require clearer knowledge of the functions of nucleotides and ectoenzymes in the cartilage extracellular matrix.

Concomitant recruitment of ERK1/2 and p38 MAPK signalling pathway is required for activation of cytoplasmic phospholipase A2 via ATP in articular chondrocytes

Extracellular ATP is a pro-inflammatory mediator involved in the release of prostaglandin from articular chondrocytes, but little is known about its effects on intracellular signaling. ATP triggered the rapid release of prostaglandin E(2) (PGE(2)) by acting on P2Y(2) receptors in rabbit articular chondrocytes. We have explored the signaling events involved in this synthesis. ATP significantly increased arachidonic acid production, which involved the activation of the 85-kDa cytosolic phospholipase A(2) (cPLA(2)) but not a secreted form of PLA(2), as demonstrated by various PLA(2) inhibitors and translocation experiments. We also showed that ATP induced the phosphorylation of p38 and ERK1/2 mitogen-activated-protein kinases (MAPKs). Both PD98059, an inhibitor of the ERK pathway, and SB203580, an inhibitor of p38 MAPK, completely inhibited the ATP-induced release of PGE(2). Finally, dominant-negative plasmids encoding p38 and ERK transfected alone into the cells impaired the ATP-induced release of PGE(2) to about the same extent as both plasmids transfected together. These results suggest that PGE(2) production induced by ATP requires the activation of both ERK1/2 and p38 MAPKs. Thus, ATP acts via P2Y(2)-purine receptors to recruit cPLA(2) by activating both ERK1/2 and p38 MAPKs and stimulates the release of PGE(2) from articular chondrocytes.

The effects of simulated microgravity on cultured chicken embryonic chondrocytes

Using the cultured chicken embryonic chondrocytes as a model, the effects of simulated microgravity on the microtubular system of the cellular skeleton, extracellular matrix, alkaline phosphatase activity, intracellular free calcium concentration and mitochondrial ATP synthase activity with its oligomycin inhibition rate were studied with a clinostat. The microtubular content was measured by a flow cytometer. The decrease of microtubular content showed the impairment of the cellular skeleton system. Observation on the extracellular matrix by the scanning electron microscopy showed that it decreased significantly after rotating, and the fibers in the extracellular matrix were more tiny and disorderly than that of the control group. It can be concluded that the simulated microgravity can affect the secreting and assembly of the extracellular matrix. In contrast to the control, there was a time course decrease in alkaline phosphatase activity of chondrocytes, a marker of matrix mineralization. Meanwhile a significant drop in the intracellular calcium concentration happened at the beginning of rotation. These results indicate that simulated microgravity can suppress matrix calcification of cultured chondrocytes, and intracellular free calcium may be involved in the regulation of matrix calcification as the second signal transmitter. No significant changes happened in the mitochondrial ATP synthase activity and its oligomycin inhibition rate. Perhaps the energy metabolism wasn't affected by the simulated microgravity. The possible mechanisms about them were discussed.

Articular cartilage calcification and matrix vesicles

There has been much research in calcium-containing crystal deposition diseases of hereditary and sporadic type. Synovial cell-induced inflammation and secondary cartilage damage are common in these diseases. In most cases of these diseases and in primary osteoarthritis, there are mineral deposits in the cartilage, mineral crystals in the synovial fluid, and aberrations of pyrophosphate metabolism.

Effects of adenosine on bacterial lipopolysaccharide- and interleukin 1-induced nitric oxide release from equine articular chondrocytes

OBJECTIVE

To determine whether adenosine influences the in vitro release of nitric oxide (NO) from differentiated primary equine articular chondrocytes.

SAMPLE POPULATION

Articular cartilage harvested from the metacarpophalangeal and metatarsophalangeal joints of 11 horses (3 to 11 years old) without history or clinical signs of joint disease.

PROCEDURE

Chondrocytes were isolated, plated at a high density (10(5) cells/well), and treated with adenosine, the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), bradykinin, or other agents that modify secondary messenger pathways alone or in combination with bacterial lipopolysaccharide (LPS) or recombinant human interleukin-1alpha (rhIL-1alpha). Nitric oxide release was measured indirectly by use of the Griess reaction and was expressed as micromol of nitrite in the supernatant/microg of protein in the cell layer. Inducible nitric oxide synthase (iNOS) activity was determined by measuring the conversion of radiolabeled arginine to radiolabeled citrulline.

RESULTS

Treatment of chondrocytes with adenosine alone had no significant effect on NO release. However, adenosine and NECA inhibited LPS- and rhIL-1alpha-induced NO release. This response was mimicked by forskolin, which acts to increase adenylate cyclase activity, but not by the calcium ionophore A23187 Treatment of chondrocytes with phorbol myristate acetate, which acts to increase protein kinase C activity, potentiated LPS-induced NO release. Adenosine treatment also significantly inhibited the LPS-induced increase in iNOS activity.

CONCLUSIONS AND CLINICAL RELEVANCE

Adenosine and the nonspecific adenosine receptor agonist NECA inhibited inflammatory mediator-induced release of NO from equine articular chondrocytes. Modulation of adenosine receptor-mediated pathways may offer novel methods for treatment of inflammation in horses with joint disease.

The extracellular ATP receptor, cP2Y(1), inhibits cartilage formation in micromass cultures of chick limb mesenchyme

We have investigated the function of the G protein-coupled receptor for extracellular ATP, chick P2Y(1) (cP2Y(1)) during development of the chick limb. cP2Y(1) is strongly expressed in undifferentiated limb mesenchyme cells but appears to be lost from cells as they differentiate, raising the possibility that the function of this receptor may be to inhibit cell differentiation. This pattern of expression was particularly striking surrounding areas of cartilage formation. We tested whether cP2Y(1) was able to regulate cartilage formation by using an in-vitro micromass model of chondrogenesis. Because limb cells in micromass culture lose expression of cP2Y(1), we have used a gain-of-function approach to demonstrate that cP2Y(1) expression can inhibit cartilage differentiation. We also demonstrate that early limb mesenchyme cells release ATP into the extracellular medium and have mechanisms to breakdown extracellular ATP. These results suggest that extracellular ATP, signaling through cP2Y(1), can modulate the differentiation of limb mesenchyme cells in vitro, and the expression pattern of cP2Y(1) suggests that this type of signaling could play a similar role in ovo.

Transient expression of P2X(1) receptor subunits of ATP-gated ion channels in the developing rat cochlea

The expression pattern of the ATP-gated ion channel P2X(1) receptor subunit was studied in the developing rat cochlea by riboprobe in situ hybridisation and immunohistochemistry. Embryonic (E12, E14, E16 and E18) and postnatal (P0, P2, P4, P6, P10 and adult) rat cochleae were examined. Both mRNA and protein localisation techniques demonstrated comparable P2X(1) receptor expression from E16 until P6 but this expression was absent at later developmental stages. P2X(1) receptor mRNA expression was localised within the otic capsule and associated mesenchyme (from E16 to P6), spiral limbus (from P0 to P6) and within the spiral ligament adjacent to the insertion of Reissner's membrane (from P2 to P6). P2X(1) receptor protein had a similar distribution based upon immunoperoxidase localisation. P2X(1) receptor-like immunoreactivity was detected in the otic capsule and the surrounding mesenchyme (from E16 to P6), spiral limbus (from P0) and epithelial cells of Reissner's membrane (from P2 to P6). The spiral ganglion neurones showed the earliest P2X(1) receptor expression (from E16 to P6). This became associated with immunolabelling of their afferent neurite projections to the base of the developing inner and outer hair cells (observed from E18 and peaking at P2). Immunolabelling of the efferent nerve fibres of the intraganglionic spiral bundle (from E18 to P6) within the spiral ganglion was also observed. The results suggest that ATP-gated ion channels assembled from P2X(1) receptor subunits provide a signal transduction pathway for development of afferent and efferent innervation of the sensory hair cells and purinergic influence on cochlear morphogenesis.

Expression of P2 receptors in bone and cultured bone cells

Extracellular nucleotides acting through P2 receptors elicit a wide range of responses in many cell types. There is increasing evidence that adenosine triphosphate (ATP) may function as an important local messenger in bone and cartilage. In this study, we used immunocytochemistry, employing novel polyclonal antibodies against P2X(1-7) receptors, and in situ hybridization, using oligonucleotide probes corresponding to P2X(2,4) and P2Y(2,4) messenger RNAs (mRNAs), to localize P2 receptors on undecalcified bone sections and on cultured osteoblasts and osteoclasts. We provide the first direct evidence that the P2X(2) receptor subtype is expressed on osteoclasts, osteoblasts, and chondrocytes. We also obtained evidence for the expression of P2X(5) and P2Y(2) receptors on osteoblasts and chondrocytes, and for P2X(4) and P2X(7) receptors on osteoclasts. Our results confirm earlier reports of P2Y(2) and P2X(4) expression in human osteoclastoma and rabbit osteoclasts, respectively, and are consistent with ATP responses observed on bone cells using electrophysiological techniques. Our novel finding that P2X(2) is expressed by osteoclasts is of particular interest. P2X(2) is the only P2 receptor subtype that requires extracellular acidification to show its full sensitivity to ATP, and our recent functional studies have shown that the stimulatory action of ATP on resorption pit formation by mature osteoclasts is amplified greatly at low pH. These findings point to fundamental new mechanisms for the local modulation of bone resorption.

Chondrocyte death is linked to development of a mitochondrial membrane permeability transition in the growth plate

In the companion article, we reported that the local phosphate (Pi) concentration triggers apoptosis in epiphyseal chondrocytes. The goal of the current investigation was to evaluate the apoptotic process in relationship to the energy status of cells in the growth plate. For these studies, we used sections of the adolescent growth plate, as well as cells isolated from the tissue. We found that there was a maturation-dependent loss of mitochondrial function in growth plate chondrocytes and these cells generated energy by glycolysis. Since treatment with the uncoupler 2,4-dinitrophenol as well as the site-specific inhibitors antimycin A and rotenone failed to elicit a further increase in the activity of the glycolytic pathway, we concluded that oxidative metabolism was minimum in these cells. Flow cytometric studies of growth plate cells and confocal microscopy of growth plate sections using the mitochondrial probes Rh123 and DiOC6(3) provided unequivocal evidence that there was loss of mitochondrial membrane potential in hypertrophic cells. Furthermore, the intrinsic fluorescence of the flavoprotein lipoamide dehydrogenase complex of the electron transport chain revealed that the mitochondria were in an oxidized state. Finally, we assessed Bcl-2 expression in these cells. Although immunohistochemical and Western blot analysis showed that the chick cells contained a low level of the anti-apoptotic protein Bcl-2, reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that transcripts were present in chondrocytes. Based on these observations, we suggest that terminally differentiated chondrocytes undergo a maturation-dependent loss of mitochondrial function. In concert with the low expression of Bcl-2, they become sensitive to signals for programmed cell death. We hypothesize that Pi triggers apoptosis in these energy-compromised cells by promoting a mitochondrial membrane transition, thereby inducing the death process.

Further characterization of ATP-initiated calcification by matrix vesicles isolated from rachitic rat cartilage. Membrane perturbation by detergents and deposition of calcium pyrophosphate by rachitic matrix vesicles

Although membrane associated enzymes such as ATPase, alkaline phosphatase, and NTP pyrophosphohydrolase in matrix vesicles (MVs) may underlie the mechanisms of ATP-promoted calcification, prior to the current investigation, the role of the MV membrane in calcification had not been addressed. In this study, various perturbations were introduced to the MV membrane in in vitro calcification systems to determine ideal conditions for ATP-initiated calcification by MVs isolated from rachitic rat epiphyseal cartilage. Membrane integrity appears to be required, since the rupture of the vesicular membrane by vigorously mixing with 10% butanol abolished calcification. In contrast, a mild treatment of MVs with low concentrations (e.g., 0.01%, which is much below the critical concentration for micelle formation) of either neutral Triton X-100 or anionic deoxycholate stimulated calcification by >2-fold, without inducing obvious changes in vesicular appearance. Fourier transform infrared spectroscopic studies were done to identify the mineral phase formed in these experiments. For the first time, rachitic MVs were shown to induce the formation of a calcium pyrophosphate dihydrate-like phase after their exposure to calcifying medium with 1 mM ATP. The integration of spectral areas indicated that calcification was enhanced by Triton X-100. The detergent effect was reversible and appeared to be not mediated through activation of ATPase, alkaline phosphatase, or ATP pyrophosphohydrolase. In contrast to neutral Triton X-100 and anionic deoxycholate, cationic cetyltrimethylammonium bromide inhibited both ATPase activity (I50=10 microM) and ATP-initiated calcification. These observations suggest that membrane perturbations can affect calcification and that the presence of NTP-pyrophosphohydrolase in MVs may play a role in the deposition of CaPPi in rachitic cartilage.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Src protein and tyrosine-phosphorylated protein profiles in marrow stroma during osteogenic stimulation

Src protein is essential for the regulation of bone turnover primarily via bone resorption because it is required in osteoclast differentiation and function. We followed temporal changes of Src protein abundance in marrow stromal cells induced to mineralize by dexamethasone (DEX), growth in cold temperature, or both. Given the tyrosine kinase function of Src and its numerous substrates, profiles of phosphotyrosine-containing proteins were followed as well. On day 11 of stimulation, specific alkaline phosphatase (ALP) activity at 30 degrees C decreased under DEX relative to 37 degrees C cultures, in accord with increased cell counts. Mineralization per well under DEX increased by 25% at 37 degrees C, whereas at 30 degrees C it increased by more than threefold regardless of the DEX stimulation. At 30 degrees C, on a per cell basis mineralization increased 2.5 and 3 times with and without DEX, respectively. Cultures at 37 degrees C showed a general drop per cell of many phosphotyrosine-containing proteins on day 3 relative to days 1 and 2 in both DEX-stimulated and nonstimulated cultures; several proteins did recover (recuperate) thereafter. On days 1 and 2, the phosphotyrosine signal was higher in several proteins under DEX stimulation; this trend became inverted after day 3. The changes in abundance per cell of Src protein (pp60src) followed a similar trend, and in addition a truncated Src molecule, p54/52src, was detected as a putative cleavage product presumably representing its carboxy terminus. The pp60src was most abundant, relative to its truncated product, in day 7 nonstimulated cultures, whereas under DEX stimulation the truncated species pp54/52src showed the highest relative abundance on days 7. At 30 degrees C, DEX stimulation accentuated the increase in Src protein on day 3, showed no change on day 7, and returned to increase Src protein on day 10. Potassium ionophorvalinomycin, considered to select against mineralizing osteoprogenitors at 30 degrees C, showed on day 10 in the absence of DEX a relative increase in truncated Src protein compared to both DEX-stimulated and nonstimulated cultures in the absence of valinomycin. On day 7 of DEX stimulation, the presence of valinomycin resulted in low p54/52src. Among phosphotyrosine-containing proteins, a 32-34 kDa band, as yet unidentified, showed the most concordant changes with mineralization induction. P32-34 decreased by DEX on days 2 and 8 and increased by low temperature alone or combined with DEX on day 3. On day 7, p32-34 did not change under DEX, but valinomycin selected cells with less phoshpotyrosine-containing p32-34. Taken together, high Src abundance at the start of osteogenic induction followed by a decrease 1 week later is probably related to energy metabolism-dependent induction of mineralization. This is in temporal accord with the increase in Src truncation and fluctuation in mitochondrial membrane potential (which affects mineralization). The reported binding of amino-terminal Src oligopeptide to p32 ADP/ATP carrier in the mitochondrial inner membrane raises the question of its possible involvement in mitochondria-regulated mineralization.

Proteoglycan breakdown from bovine nasal cartilage is increased, and from articular cartilage is decreased, by extracellular ATP

The addition of ATP, but not ADP or AMP, to the culture media of bovine nasal cartilage explants caused an acceleration in the rate of proteoglycan loss from the tissue. The ATP-stimulated loss of proteoglycan was not inhibited by the IL1-receptor antagonist protein, but was partially inhibited by the presence of ADP or AMP. The proteolytic events resulting from the presence of ATP were found to be similar to those following treatment with IL1, in that inhibitors of the cysteine-peptidase cathepsin B, serine-proteinases with trypsin-like specificity, and of some of the matrixins, could all prevent proteoglycan loss, which was mediated, at least in part, by the action of 'aggrecanase'. In contrast to its effects on nasal cartilage, ATP inhibited basal and stimulated proteoglycan release from articular cartilage. Both ADP and AMP had no effect on proteoglycan release in articular cartilage but enhanced the response to ATP when added concurrently. We conclude that extracellular ATP, probably acting via P2-purinoceptors, stimulates proteoglycan breakdown from bovine nasal cartilage and thus, may have a role in diseases which primarily involve destruction of non-articular cartilage. Extracellular ATP has, in contrast, a chondroprotective effect on bovine articular cartilage.

Purinergic transmitters inhibit bone formation by cultured osteoblasts

Adenosine triphosphate (ATP) and other purinoceptor agonists cause a transient rise in [Ca2+]i in cultured osteoblast-like cells and have a mitogenic effect, as does parathyroid hormone (PTH), and there is evidence that ATP and PTH can act synergistically on osteoblasts. The likelihood that nucleotides, acting through purinoceptors, are important local factors in bone remodeling is therefore considerable. However, their effect on bone formation is unknown. We recently developed a culture system in which appositional bone formation occurs only in narrow grooves cut in a substratum. We have used this as an assay to measure the effects of ATP (50 and 500 mumol/L), ATP gamma S (20 mumol/L), 2-MeSATP (2 and 20 mumol/L), uridine triphosphate (UTP) (0.2, 2, and 20 mumol/L), adenosine (20 mumol/L), bovine PTH (0.25 and 0.5 IU/mL), rat PTH1-34 (10(-8) and 10(-7) mol/L), and rat PTHrP1-40 (10(-9) and 10(-8) mol/L) on bone formation by rat calvarial osteoblasts. The culture medium was renewed 3 times/week (every 2 or 3 days), and the number of bone loci and length and area of Alizarin red-stained mineralized bone formed in the grooves of each specimen in 16-29 days were measured. Compared with controls, ATP gamma S, 2-MeSATP, and ATP reduced the amount of bone formed in a 2-3 week culture period. Adenosine had no effect, and UTP either had no effect or at 2 mumol/L stimulated bone formation. PTH and PTHrP completely abolished bone formation in 4 week cultures. Our findings are consistent with evidence for more than one P2 purinoceptor subtype in bone, and show for the first time that the effect of ATP on appositional bone formation by osteoblasts in vitro is, like PTH and PTHrP, inhibitory.

Interaction of annexins IV and VI with ATP. An alternative mechanism by which a cellular function of these calcium- and membrane-binding proteins is regulated

Annexin VI from porcine liver can be photoaffinity-labeled with 8-azido-[gamma-32P]ATP in a concentration-dependent, saturable manner. The extent of labeling varied with the concentration of calcium. The dissociation constant for the nucleotide was found to be in the range reported for ATP-binding proteins. The ATP analog, 2'-(or 3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-triphosphate, also bound to AnxVI, as indicated by shift in its fluorescence spectra in the presence of protein. Any significant 8-azido-ATP or TNP-ATP binding was not observed with AnxIV. ATP modulated the binding of AnxVI to erythrocyte membrane and increased the Ca2+ concentration required for half-maximal binding of AnxVI to F-actin.

Extracellular ATP modulates [Ca2+]i in retinoic acid-treated embryonic chondrocytes

When treated with low doses of retinoic acid (RA), cephalic chondrocytes of the chick embryonic sternum mature and express phenotypic characteristics of postmitotic hypertrophic cells. In concert with these maturation-dependent changes, cells release adenine nucleotides into the culture medium. To ascertain if these compounds modulate chondrocyte function, we challenged chondrocytes with nucleotides and measured one determinant of the signal transduction pathway, intracellular Ca2+ concentration ([Ca2+]i). In the presence of micromolar concentrations of ATP, there was a dose-dependent elevation in chondrocyte [Ca2+]i; ADP caused a small but significant rise in the peak [Ca2+]i response. We found that the change in the [Ca2+]i response is linked to retinoid-dependent maturation of chondrocytes. Thus the [Ca2+]i rise was dependent on the RA concentration and treatment time. Immature caudal chondrocytes, cells that were not affected by RA, were used as control cells for this study. When treated with ATP, these cells did not exhibit a [Ca2+]i response. Although the purinergic subtype receptor was not characterized, the observation that cells responded to ATP and ADP but were refractory to AMP and adenosine suggested that P2 purinoceptors were expressed by chondrocytes. Because, during the same culture period, chondrocytes exhibited many of the unique characteristics of the terminally differentiated cell, the acquisition of purinergic receptors represents a new feature associated with expression of the mature phenotype. Finally, to ascertain if the ATP-dependent response was due to release of Ca2+ from intracellular stores, cells were treated with thapsigargin. Since this compound significantly reduced the [Ca2+]i signal, we concluded that the ATP response is mediated by release of cation, from the endoplasmic reticulum.

Evidence of the presence of a specific ATPase responsible for ATP-initiated calcification by matrix vesicles isolated from cartilage and bone

Accumulating evidence indicates that calcification by isolated mammalian matrix vesicles (MVs) can be initiated by ATP. Since ATP can be hydrolyzed by either a specific ATPase or by nonspecific alkaline phosphatase (ALP), it remains to be established whether ATPase or ALP mediates ATP-initiated Ca and Pi deposition. To support the hypothesis that specific ATPase is responsible for ATP-initiated calcification by MVs isolated from mammalian cartilage and bone, the effects of ATP analogs, ALP substrates, and specific inhibitors on ATP hydrolysis and ATP-initiated calcification were compared between intact MVs and monoclonal antibody affinity-purified MV ALP. ATP analogs such as ADP and AMP exerted marked inhibitory effects on both [gamma-32P]ATP hydrolysis and ATP-initiated calcification by intact MVs, whereas phosphomonoesters such as beta-glycerophosphate or phosphoethanolamine had no effect. In contrast to intact MVs, purified MV ALP failed to calcify, and its [gamma-32P]ATP hydrolytic activity was readily inhibited by phosphomonoesters. Additionally, [gamma-32P]ATP hydrolysis by purified ALP in contrast to that by intact vesicles was completely inhibited by l-tetramisole, a specific inhibitor of ALP, suggesting a loss of specific ATPase during purification. Vanadate inhibition of ATP hydrolysis by purified ALP can be decreased by increasing ATP concentrations. On the contrary, ATP concentrations did not affect vanadate inhibition of ATP hydrolysis by intact MVs if ALP activity was blocked by l-tetramisole. These observations, therefore, suggest that: 1) a portion of [gamma-32P]ATP hydrolysis by MVs is attributable to a specific ATPase, whereas the remaining activity is due to ALP; and 2) a specific ATPase, but not ALP, is responsible for ATP-dependent Ca- and Pi-depositing activity of MVs isolated from bone or cartilage.